Fitting patio doors comes down to the threshold. Get it level and fully supported, and everything after it follows. Set the frame, pack and fix it, check the diagonals, then hang and adjust the doors. An out-of-level threshold shows up later as a door that drags or will not seal.
Three things need settling before the old door comes out, and two of them happen at the ordering stage rather than on the day.
The regulations. Replacing an external door is notifiable work under the Building Regulations. Either a FENSA or CERTASS registered installer self-certifies it, or you submit a building notice to your local authority Building Control yourself. We supply doors rather than fit them, so if you are fitting your own, the notification is yours to make. Our guide to removing a window sets out both routes in full.
The threshold. Patio doors come with a choice of threshold, and the right one depends on what is outside the door and how the ground falls away from it. That choice is made when you order, not when you fit. If you have not ordered yet, settle it now.
The measurements. The door is made to your opening with a fitting tolerance, so measure in three places across the width and three up the height, and work to the smallest of each. Our measuring guide covers patio and sliding doors specifically.
A window sits in its opening and stays there. A patio door has a moving leaf that runs along the bottom of the frame hundreds of times a year, and everything that leaf does is measured against the threshold.
If the threshold is out of level, a hinged door swings open or closed on its own and the lock will not line up. A sliding door drags at one end of its travel. Neither problem improves with adjustment, because adjustment moves the door relative to a frame that is itself wrong.
The threshold also has to be supported along its whole length, not just at the ends. It carries the weight of the leaves, and an unsupported middle section flexes every time someone steps over it. Over a few years that flex is what loosens the seal at the bottom of the door.
And it has to shed water. The ground outside should fall away from the door, and any drainage channel or gully in front of it needs to stay clear. A threshold that sits in a puddle is a threshold that will let water in eventually, whatever the seal is rated for.
Get those three things right, level, supported and drained, and the rest of the job is straightforward.
Step five is the one that separates a door that works from one that never quite does. A frame can be plumb on every upright and still be out of square, and a spirit level will not tell you. Two diagonals that match will.
Step four matters almost as much. Every fixing needs solid packing directly behind it, because a screw pulled tight against an unpacked gap bows the frame inward. On a patio door that bow is enough to stop the lock engaging.
Most of our patio doors come with a fix mode as standard, which holds the leaf in position while you work. Use it during the fitting rather than propping the door against a wall.
Leave the adjustment until after the sealing. Seals and packers settle slightly in the first day, and a door adjusted before that settles needs adjusting again.
Note: Contact our customer support for more help on fitting patio doors.
On a pair of French doors the two leaves do different jobs. The main sash opens on a handle. The secondary sash is held by latches at the top and bottom, and in a double door without a centre post it is what the main sash closes against.
That changes the order of work. Align and secure the secondary sash first, so it sits square in the frame and its latches drop cleanly into their keeps. Then hang the main sash and set it against that.
Work the other way round and you end up adjusting the main sash to meet a secondary sash that is itself out of line. The result is a door that closes but does not seal, usually at the top corner.
Check the gap down the meeting stile with the doors closed. It should be even from top to bottom. A gap that tapers means the frame is out of square, and that goes back to the diagonals rather than to the hinges.
Installing a single French door is the same job without the second sash. It is the most manageable of the patio door types for a competent joiner, which is why it is often the one people fit themselves.
A sliding patio door works differently from a hinged one, and the difference matters during fitting. Turning the handle raises the leaf slightly off its seals before it moves, and sets it back down when you close it.
That lift is what makes a large panel glide easily and what lets the closed door seal tighter than a conventional slider. It also means the running rail has to be dead level along its full length. A rail that dips in the middle gives you a door that runs freely at the ends and drags in the centre.
Support the rail continuously while you bed it. Any void under it will show up once the weight of the glazed leaf is on it, and by then the frame is fixed.
Check the drainage at the rail before you finish. A sliding door threshold has drainage channels that carry water out, and if they are blocked by mortar or packing during installation, water sits in the track.
Slide the leaf its full travel before you seal up. It should move with the same effort at both ends and lift and drop cleanly at the handle. If it does not, the fix is the frame, not the hardware.
Seal the joint between frame and wall on both faces. Outside takes a low-modulus silicone that stays flexible as the frame moves. Inside takes paintable acrylic, and it should be the tighter of the two.
Do not fill the gap solid with expanding foam and leave it at that. Foam insulates, but it is not the weather seal and it is not vapour tight, and an unsealed foam joint breaks down in the open air. Our guide to resealing windows covers what each joint is doing.
At the threshold, check that the seal does not bridge the drainage path. It is an easy mistake: a neat bead of silicone across the front of the threshold looks like good work and stops the water getting out.
Make good the internal reveal last, once the door has been opened and closed a few times and nothing has shifted.
Give it a day, then check three things. Does the door close without being pushed? Does the lock engage without lifting the handle hard? Is the gap even all the way round the leaf?
If the answer to any of those is no, adjust the hinges or the rollers rather than forcing it. Our guides to adjusting french doors and adjusting patio doors cover the hardware.
If adjustment does not fix it, the frame is out. That is worth knowing early, while the internal reveal is still open and the fixings are still accessible.
On timber and alu-clad doors, check the fitting again after the first winter. Timber moves with the seasons and a door that was perfect in September can need a quarter turn in March.
We supply doors rather than fit them, and a single garden door or a set of French doors is within reach of a competent joiner. Four cases where it is not a DIY job:
All our doors are UK-CA marked and made to order in our own factory, and installation manuals and videos for our products are on the site. What we cannot do is sign a FENSA certificate for work we did not carry out, so plan the notification route before the old door comes out.
Good luck with it. If the diagonals do not match when the frame is propped, stop and fix that before you put a single fixing in.
